Effects of UV Irradiation and Plasma Treatment on a Polystyrene Surface Studied by IR−Visible Sum Frequency Generation Spectroscopy

IR−visible sum frequency generation (SFG) spectroscopy was used to study surface modification of polystyrene by its exposure to a UV light source or plasma. It was found that the polystyrene surface underwent dramatic changes after exposure to these treatments, as evidenced by marked changes in the surface SFG spectra. Before the treatments, the surface spectrum showed a pronounced peak at 3068 cm-1 which is characteristic of the symmetric stretch of the aromatic C−H of polystyrene. This peak decreased markedly, and other vibrational bands associated with the CH2 and CH3 groups appeared after the treatments. The observed spectral changes provided direct evidence of surface reactions involving the aromatic ring. In addition, our data showed that the degrees of oxidation of the polystyrene surface were different with the two processes. The oxidation to a higher oxidation state, resulting in the formation of carbonyl/carboxyl species, was observed with plasma treatment but not with UV irradiation. This diffe...